1. Field of the Invention
The present invention relates to oil and gas well proppants and, more particularly, to sintered proppants containing clay as a major component, a method of making such proppants, and to a method of maintaining a fracture in a subterranean formation in a propped condition by utilizing such proppants.
2. History of the Prior Art
Oil and natural gas are produced from wells having porous and permeable subterranean formations. The porosity of the formation permits the formation to store oil and gas, and the permeability of the formation permits the oil or gas fluid to move through the formation. Permeability of the formation is essential to permit oil and gas to flow to a location where it can be pumped from the well. Sometimes the permeability of the formation holding the gas or oil is insufficient for economic recovery of oil and gas. In other cases, during operation of the well, the permeability of the formation drops to the extent that further recovery becomes uneconomical. In such cases, it is necessary to fracture the formation and prop the fracture in an open condition by means of a proppant material or propping agent. Such fracturing is usually accomplished by hydraulic pressure, and the proppant material or propping agent is a particulate material, such as sand, glass beads or ceramic particles, which are carried into the fracture by means of a fluid.
Spherical particles of uniform size are generally acknowledged to be the most effective proppants due to maximized permeability. For this reason, assuming other properties to be equal, spherical or essentially spherical proppants, such as rounded sand grains, metallic shot, glass beads and tabular alumina, are preferred.
In practice, in deep wells, where high pressures are encountered, e.g., above about 700 Kg/cm.sup.2 (10,000 psi), the foregoing specifically mentioned proppants are either entirely ineffective or do not exhibit desired permeability. Examples of prior art proppants and their use are found in U.S. Pat. Nos. 2,950,247, McGuire et al; 3,026,938, Huitt et al; 3,126,056, Harrell; 3,497,008, Graham et al; 3,976,138, Colpoys et al; and 4,068,718, Cooke et al. One of the better proppants useful at high pressures, disclosed in U.S. Pat. No. 3,976,138, is alumina. However, even alumina, as disclosed in U.S. Pat. No. 3,976,138, has reduced permeability at pressures in excess of 350 Kg/cm.sup.2 (5,000 psi).
As disclosed in U.S. Pat. No. 4,068,718, sintered bauxite unexpectedly has a permeability which is superior to the previously mentioned proppant materials at pressures as high as 700 Kg/cm.sup.2 (10,000 psi) or higher.
Unfortunately, the sintered bauxite material actually used in making the measurements disclosed in U.S. Pat. No. 4,068,718 does not have a desired spherical shape, which would provide high permeability.
The prior art sintered bauxite particles may be produced in spherical shape as described in R. J. Seider's commonly assigned, copending U.S. patent application Ser. No. 252,491, filed Apr. 8, 1981, as a continuation of U.S. patent application Ser. No. 969,122, filed Dec. 13, 1978. However, such proppants, although extremely useful under high pressure conditions, over about 700 Kg/cm.sup.2 (10,000 psi), are costly. The cost of the prior art sintered bauxite proppant for wells of intermediate pressures, between about 350 and 700 Kg/cm.sup.2 (5,000 and about 10,000 psi), may not be economically justified. The present invention provides proppants aptly suited to use under pressures of up to about 700 Kg/cm.sup.2 (10,000 psi), which are more economical and have lower specific gravities and bulk densities, which would benefit the user, in that fewer pounds of proppant per cubic foot of fracture would be required. Handling, e.g., pumping of slurries of lower density material, is also made easier.